1. Field of the Invention
The present invention relates to a control device which controls a torque generated in an electric rotating machine by controlling a voltage outputted from a power inverter circuit to the machine, and also relates to a control system having the control device and the power inverter circuit.
2. Description of Related Art
A control device for a three-phase motor has been used. For example, to control three phase currents, actually flowing through respective phase windings of the motor, to instructed values under the feed-back control, this device performs a pulse width modulation (PWM) control for the motor. In this PWM control, instructed values of phase voltages outputted from an inverter to the phase windings of the motor are calculated, and switching elements of the inverter are operated based on the difference between the level of a predetermined carrier wave and the instructed level of the phase voltage. Therefore, the phase voltage applied to each phase winding can be controlled to the instructed value, so that the phase current flowing through each phase winding can be controlled to a desired value.
However, when the motor is set in a high rotation speed region so as to generate a high torque at a high rotation speed, the amplitude of the instructed phase voltage is increased so as to be equal to or higher than a half of an input voltage of the inverter. Therefore, in the high rotation speed region of the motor, it is difficult to control the voltage, actually outputted from the inverter, to the instructed value under the PWM control.
To perform the control for the motor in the high rotation speed region, the rectangular wave control has been used in place of the PWM control. In this rectangular wave control, the on-off cycle of each switching element of the inverter is set to be substantially the same as the rotation cycle of the motor expressed by electrical angle. The on-off cycle is defined as the reciprocal number of the on-off repetition frequency. However, the ratio (hereinafter, called voltage utilization ratio) of a controlled voltage outputted from the inverter to a voltage inputted to the inverter becomes large in the rectangular wave control, as compared with the maximum voltage utilization ratio of 1/2 in the PWM control. The voltage utilization ratio in the PWM control is maximized when the amplitude of the instructed voltage becomes 1/2 of the voltage inputted to the inverter.
Therefore, when one of the PWM control and the rectangular wave control is switched to the other one in response to a change of the torque required of the motor, the voltage utilization ratio is discontinuously changed. In this case, the torque generated in the motor becomes unstable due to this discontinuous change.
To continuously change the voltage utilization ratio even when the operation of the motor is shifted to the high rotation speed region, Published Japanese Patent First Publication No. H09-047100 discloses a control device for a motor. In this device, d- and q-axis components of an instructed voltage defined on the dq rotational coordinate system are calculated from an instructed current, determined based on a target torque, for the current feed-back control. When the amplitude of the controlled voltage outputted from an inverter to a three-phase motor is equal to or higher than a half of the voltage inputted to the inverter, the inverter is operated according to either a phase of the controlled voltage, calculated from the instructed voltage, or one of pulse patterns of a voltage signal stored in a read only memory (ROM). Each pulse pattern is formed to be appropriate to one value of the voltage utilization ratio. In other words, when the vector norm (i.e., vector length) of the instructed voltage is equal to or higher than a predetermined value, the inverter is operated according to the pulse pattern of the ROM corresponding to the vector norm. Therefore, the voltage utilization ratio in the control based on the pulse patterns of the ROM can be increased so as to approach the value obtained in the rectangular wave control.
The pulse pattern of the ROM is designed according to a motor model in which a motor is typically designed by adjusting parameters specifying characteristics of the motor. However, the performance of one motor actually manufactured differs from that of another motor, so that the pulse pattern of the ROM is not appropriate to all motors actually manufactured. Further, even when the pulse pattern is appropriate to one motor actually manufactured, the performance of this motor is changed with time due to aging. Moreover, the control device performs a maximum torque control for the motor to which the voltage based on the pulse pattern is applied. In this maximum torque control, the maximum torque can be generated in the motor by supplying the minimum current to the motor. In this case, even when the pulse pattern of the ROM is designed so as to be appropriate to the maximum torque control, the device cannot sometimes perform the maximum torque control for the motor.
For example, the control device selects the control based on the pulse pattern of the ROM or the control based on the voltage phase in the PWM control to perform the maximum torque control under the selected control. When the control selection is changed, the output voltage of the inverter according to the control of the pulse pattern of the ROM is differentiated from the output voltage of the inverter according to the control of the voltage phase in the PWM control. Therefore, each time the selection of the control based on the pulse pattern of the ROM or the control based on the voltage phase in the PWM control is switched to another one, the output voltage of the inverter is discontinuously changed, and the controllability in the control of the motor is lowered.